Methods for making an optical preform via vapor deposition are known. These methods, however, yield optical preforms lacking geometric and optical uniformity.
One such method, for example, is disclosed in U.S. Pat. No. 4,741,747. This patent identifies as important deposition parameters reproducibility, the yield of the chemical reaction between the glass-forming precursors, and the deposition rate and the optical and geometric homogeneity of the material deposited on the interior of the substrate tube.
During the deposition process, however, so-called “end tapers” (also called deposition zones) where the optical and geometric properties are not sufficiently constant, can form at both ends of the substrate tube. It is known that the length of such end tapers may amount to about 15 percent of the total length of the substrate tube. Such a taper would lead to an axially non-uniform cross-section of the core after an optical fiber has been drawn from the preform. In particular, the taper forms an area in which the optical and/or geometric properties of the preform are not uniform. Such non-uniformity will lead to degradation of the transmission qualities of the optical fiber. Such taper areas in the preform cannot be used to manufacture quality optical fiber and, consequently, that portion of the preform becomes waste (as is any optical fiber drawn therefrom). Because such taper areas make up a significant part of the preform tube, the total fiber length that can be obtained from a preform is rather limited.
To prevent the occurrence of taper in an optical preform, U.S. Pat. No. 4,741,747 proposes to move the energy source (i.e., a plasma) non-linearly as a function of time in the area of at least one of the points of reversal or to change the power of the plasma over the length of the substrate tube as a function of time. Previously, a so-called “stationary velocity profile” of the resonator was used for depositing the layers on the interior of the substrate tube.
U.S. Pat. No. 5,188,648 relates to a method for reducing the taper area wherein the reciprocating movement of the plasma is interrupted at the point of reversal at the supply side of the glass-forming precursors. The duration of such interruption is set to effectively reduce the taper area at the so-called gas side of the substrate tube. This patent is specifically directed to reducing the geometric taper of the core of the optical preform.
U.S. Pat. No. 5,145,509 relates to a method for reducing the geometric taper wherein a glass rod is placed in the center of the substrate tube. The radius of the rod is set so that it amounts to maximally 0.67 and minimally 0.2 times the internal radius of the glass substrate tube (i.e., between about 0.2× and 0.67×the internal radius of the glass substrate tube). The glass rod is removed from the interior of the substrate tube upon completion of the deposition process. Thereafter, the hollow substrate tube is contracted into a solid preform under elevated temperature conditions.
U.S. Pat. No. 4,944,244 relates to a method for manufacturing optical preforms wherein the power of the energy source is continuously controlled during the deposition process based on a signal that is a function of, among other factors, the degree to which deposition of glass layers on the interior of the substrate tube takes place.
U.S. Patent Publication No. 2005/0041943 relates to a deposition method wherein the plasma is moved along a hollow substrate tube and is changed in a first end region adjacent to a reversal point, both as a function of time in the deposition process and as a function of position in the first end region. The end point of the first end region coincides with the reversal point, and the starting point is located further away from the reversal point than the deceleration point. The first end region has a length that suffices to reduce the taper in the preform.
European Patent Publication No. EP 1,396,554 relates to a system for plasma-activated chemical vapor deposition. This publication discloses feeding electromagnetic power of a microwave from antennas provided in an inner periphery portion of an annular waveguide into a reaction chamber disposed inside the waveguide. This allows plasma to be generated in the interior of the reaction chamber, thereby forming a film.
U.S. Patent Publication No. 2003/0115909 relates to a method of making an optical fiber preform assembly. The disclosed method includes forming a plasma inside a tube, thereby forming a plasma zone, and introducing a flow of at least one precursor suitable for forming glass into the plasma zone, wherein the flow comprises eddy diffusion of the flow of the precursor.
Each of the foregoing patents and patent publications are hereby incorporated by reference in their entirety.
The prior art discloses methods for manufacturing preforms wherein the optimization of the geometric taper has led to the formation of an optical taper, and vice versa. In this regard, geometric taper refers to reduced (or increased) deposition thickness at the ends of the optical preform. Optical taper refers to the outer part of an optical preform in which an optical property (e.g., refractive index) deviates from its optical set point. As will be understood by those having ordinary skill in the art, such optical deviation worsens toward the outer ends of the optical fiber preform.
There is a need, however, for a method for making optical fiber preforms that possess improved geometric and optical uniformity.